Install casks via Caskfile

`brew bundle` could be used for casks as well.

I hope this change will help with:
- consistency
- showing users `brew-casks` is a plugin on top of Brew, not a separate
package manager.
- removing the obsolete IMHO `installcask` function

I would remove the `STDERR` redirection as well (`2> /dev/null`), but thought I'd discuss it first.